1. Field of the Invention
The present invention relates to digital time overcurrent relay devices, and, in particular, to matching selected overcurrent characteristic curves.
2. Description of the Related Art
Relay devices, commonly referred to as relays, are widely used in many applications, such as providing overcurrent protection for industrial and utility power systems. Typical uses include protection of utility and industrial feeders from phase and ground faults, and short circuit and overload protection for transformers, AC machines, transmission lines, or facilities with medium voltage switchgear. Such relays typically include both overcurrent detection circuitry that generates a trip control signal after overcurrent is detected, and a trip circuit to energize a breaker trip circuit when the trip control signal is generated.
The overcurrent detected may be based on a time or instantaneous overcurrent (known as TOC and IOC, respectively). Thus, for example, whenever a TOC condition or event is detected, the overcurrent detection circuitry generates a trip control signal, which is applied to the trip circuit. When the trip circuit receives the trip control signal, the trip circuit causes an appropriate circuit breaker to trip, thus protecting the device or system in which an overcurrent has been detected.
Relays typically provide several selectable TOC characteristic responses, each of which corresponds to one of a family or plurality of time overcurrent characteristic curves, or TOC curves, which plot time versus current. Each curve of a family of TOC curves is typically an inverse-time characteristic curve. See, e.g., IEEE Standard Inverse-Time Characteristic Equations for Overcurrent Relays, Designation C37.112-1996 (available from the IEEE, 1-800-678-IEEE, 445 Hoes Lane, P.O. Box 1331, Piscataway, N.J. 0885-1331, USA), which defines the inverse-time characteristics of overcurrent relays.
To implement an inverse-time TOC curve, relays typically perform RMS sensing or are provided with the RMS current, and begin to integrate current over time after an initial threshold, or "pick-up," current level is exceeded. For example, after pick-up occurs, the relay may trip if an RMS current of 2 Amps is detected for 1.5 seconds, or, alternatively, if an RMS current of 20 Amps is detected for 0.15 seconds. Thus, a larger average RMS current generally causes the relay to trip, due to a detected TOC condition, in a shorter time.
Many families of pre-established inverse-time TOC curves have been published. It is often desirable for a relay to offer at least one set of selectable TOC characteristic responses that conform to an established set of published TOC curves. A typical TOC curve family comprises eleven TOC curves, for example. Each curve of a TOC curve family typically corresponds to a different "time dial" setting, such as time dial settings 1/2, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. A higher time dial setting slows down the time overcurrent response, and vice-versa. Each curve of the family is plotted in terms of time versus current, where current is specified in multiples of pick-up setting. In practical implementations, each curve is typically plotted from a point just past a unity pick-up multiple setting, e.g. a multiple of 1.5 pick-up settings.
The curves of the family have similar, but not necessarily identical, shapes--for example, curves at the higher time dial settings are typically somewhat more linear than curves at the lower time dial settings. One reason for the difference in curve shapes within a curve family is that in electro-mechanical (analog) relays, physical devices like springs, and other related mechanical aspects of such relays, cause curve shape to change as time dial settings are changed.
There is a need to replace electro-mechanical relays with digital relays that do not require the routine calibration required by electromagnetic relays. Digital relays are also less susceptible to vibration than electro-mechanical relays. It is desirable, therefore, to provide digital relays that provide multiple selectable TOC curves, which conform to an established, or "target," set of TOC curves, for example, a published curve family, or curve family implemented by an existing device or set of devices. Digital relays typically implement the integration function by running an accumulator which accumulates values calculated from sampled RMS current values. If the digital relay provides TOC curves that conform to established set of published or target TOC curves, then existing electro-mechanical relays that may be replaced with digital relays without the necessity of recoordinating the entire system.
Digital relays typically implement a given TOC curve by storing, typically in a ROM look-up table or other type of memory or storage device, a set of data points that approximate the curve. However, for multiple curves of a family of curves, the memory requirements associated with storing data points for all the curves can be prohibitively expensive or otherwise undesirable.